1. Field of the Invention
The present invention relates to a method for introducing gases to liquids, and a stirring system for carrying out said method.
2. Discussion of the Background Art
The known stirring systems for introducing gases to liquids are of basically two types:
1. Stirring systems with external introduction of the gas where the gas is supplied via a simple tube or dispersion ring, having openings which open out in the lower part of the stirring vessel. The exiting gas is distributed into the liquid with the aid of a stirrer (e.g., a disc-type stirrer, an inclined plate or propeller-type stirrer, or a continuous stirrer). This system has the fundamental disadvantage of requiring a pump which must at least overcome the pressure of the liquid column. Further, if the gas stream is shut off, the liquid can penetrate into the gas dispersing system. Another disadvantage is that the stirrer can be bypassed, particularly at high gas flow rates; i.e., the stirrer is unable to completely disperse the gas exiting from the gas feed line. The result is pulse-loading of the entire stirring system, and thus elevated wear.
2. Self-suction stirring systems which have the advantage that there is lower pressure on the inclined stirring surfaces during stirring. If the stirring structure is configured as a tube, and gas is fed to it, the self-inducing action on the stirrer surfaces will cause the gas to pass outward. (See the description of the Ystral system and M. Zlokarnik, "Ruehrtechnik", in Ullmanns Encyklpaedie der technischen Chemie, 4th Ed. Vol. 2, pub. Verlag Chemie, Weinheim, FRG, 1972, pp. 259 ff., particularly pp. 275-278). The Ystral system is based on the fact that at a suitable stirrer rpm a decrease in pressure develops in the middle of the stirring mechanism, such that the gas can exit the tube.
Self-inducing systems require relatively high stirring power in order to achieve adequate gas dispersion. This is particularly true in cases in which there is a substantial vertical distance between the surface of the liquid and the stirrer. There is a limit on the maximum height difference, this limit being imposed by the laws of physics.
It is also known to provide a mixing vessel with a draft tube and a propeller stirrer, with the stirrer disposed within the guide tube, in order to achieve improved axial flow within the stirring vessel. Such systems are employed, e.g., for producing and maintaining solid-liquid and liquid-liquid interaction systems (See V. M. Uhl and J. B. Gray, "Mixing", Vol. II, pub. Academic Press, New York, London, 1967, pp. 247 ff.). Draft tubes are ordinarily used when it is desirable to substantially suppress the formation of a large funnel-type vortex in a cylindrical container, i.e., when it is desirable to avoid uptake of gas into the liquid as much as possible in the presence of good axial intermixing. Thus, the prior art suggests that stirring systems with draft tubes are not at all suitable for introducing gases to liquids.
Particular problems in introducing gases to liquids occur when the capacity of the liquid for the gas is relatively small or when the gas-liquid system tends to coalesce. In such cases, in order to employ the gas it is necessary to capture the gas which has not been absorbed by the liquid, and to reintroduce such gas to the liquid. Problems always arise when the liquid to which the gas is to be introduced substantially changes its material properties during the introduction of the gas.
Although numerous stirring systems have been proposed, a need continues to exist for an efficient stirring system for introducing gases to liquids.